The present invention relates to a lighting device including an LED.
Light-emitting diodes (LEDs) are increasingly used as light sources that can replace incandescent light bulbs and fluorescent lamps. Since LED elements are small, various types of LED elements may be combined to form a variable color unit.
A lighting device including an LED is often designed to have a general color rendering index of 80 or greater to satisfy the indoor lighting conditions specified by Japanese Industrial Standard (JIS) Z9110:2004. The general color rendering index Ra is used to quantitatively evaluate the level a color can be truly reproduced under a subject light source in comparison with a reference light. Thus, when a light source has a high general color rendering index Ra, the light source ensures reproduction of a color appearance under the reference light. However, when the plane of a paper appears to be colored by light of various colors, the general color rendering index Ra cannot be used to evaluate the light source. Thus, even when the general color rendering index Ra is high, the plane of a paper may appear to be colored. This may decrease the contrast between characters and the background and make it difficult to read characters.
Accordingly, it is desirable that the whiteness of a paper plane be quantitatively evaluated to provide light that is suitable for reading. Japanese Laid-Open Patent Publication No. 2002-13982 describes a method for calculating the whiteness level from the spectrum of a fluorescent lamp. The whiteness level is used as an index quantitatively indicating the whiteness of a paper plane under lighting. In Japanese Laid-Open Patent Publication No. 2002-13982, a lighting device uses a fluorescent lamp as a light source, and the chromaticity range is limited to the proximity of 5000 K and the proximity of 8000 K in correlated color temperature.
However, the lighting device of the publication uses the fluorescent lamp as the light source and has a spectrum that differs greatly from a lighting device that uses an LED as a light source. Thus, it is difficult to directly apply an LED element to the above light device. Further, as described above, when a colored light is evaluated with only the general color rendering index Ra, the paper plane may appear to be colored making it difficult to read characters. When increasing only the whiteness of a paper plane under lighting, a low general color rendering index Ra may lower the reproductivity of chromatic colors and result in colors appearing strange to a user.